Laser cutting of semiconductor wafers comprising a plurality of semiconductor dice is commonly used as an efficient and accurate die-singulation process. Such laser cutting may be conducted by cutting completely through the wafer in a process called dicing or by forming grooves in the wafer in a process called grooving. The wafer that needs to be singulated is mounted onto a special mounting tape, which may be in the form of a polymer adhesive mounting tape that holds the individual dice in place during the singulation process. Additionally, the singulated dice are also held in place after laser singulation when the wafer and the tape are transferred to the next tool for further processing. There are many different types of adhesive mounting tape available depending on the application. Many of these tapes are curable by exposure to ultra-violet (“UV”) light, so that the adhesive forces between the mounting tape and the singulated dice are reduced after the said exposure. This allows the singulated dice to be removed more easily from the mounting tape.
When UV laser radiation (such as by applying a laser beam having a wavelength of 355 nm) is used in the die singulation process, it is a challenge to combine the desirable UV-curing properties with sufficient resistance to UV laser radiation to avoid damage to the UV-resistant mounting tape during laser cutting. Although some UV-resistant mounting tapes exist that combine these characteristics (being both UV curable and UV laser-resistant), the range of available UV-resistant mounting tapes is very limited and the costs of these UV-resistant mounting tapes are very high, thereby limiting the use of UV dicing for the purpose of mass production.
As a result, semiconductor manufacturers resort to the use of these exotic tapes only for applications that can tolerate the additional cost of the UV-resistant mounting tape. Alternatively, solutions based on green or infrared laser have been developed, but they are inferior when compared to a UV laser singulation solution, since they result in larger kerf-widths, more recast and larger burrs.
FIG. 1 illustrates a semiconductor wafer 10 mounted on an adhesive mounting tape 12 and a number of cutting lines 14 along which a laser beam 16 moves to cut the semiconductor wafer 10. The adhesive mounting tape 12 is in turn attached to a rigid frame 18 which supports the adhesive mounting tape 12 for fixedly securing the semiconductor wafer 10 during laser cutting and for transportation of the semiconductor wafer 10.
During the singulation process, the laser beam 16 should cut the semiconductor wafer 10 all the way until the edge of the semiconductor wafer 10. However, the actual cutting lines 14 typically extend onto the adhesive mounting tape 12. If the cutting lines 14 extend outside a perimeter of the semiconductor wafer 10, or if the laser beam 16 inadvertently exceeds the wafer edge by a small distance, the laser beam 16 would impinge into and deposit its laser energy into the adhesive mounting tape 12 and this may damage the adhesive mounting tape 12. Accordingly, the adhesive mounting tape 12 should be able to resist the focused laser beam 16 without incurring excessive damage. For UV laser cutting applications, this requires expensive custom-made mounting tapes. The laser beam 16 may comprise a single laser beam, or may comprise multiple laser beams. The latter is adaptable to cut along multiple cutting lines 14 simultaneously.
If special UV-resistant adhesive mounting tapes are to be avoided, the singulation machine has to be sufficiently accurate so that laser-cutting stops exactly at the boundary of the semiconductor wafer 10. This is difficult to achieve in practice, and even if feasible, would place additional requirements on the accuracy and performance of the singulation machine. If either the adhesive mounting tape is punctured, the singulation machine cuts over the edge of the semiconductor wafer 10, or where a non-cut portion appears at the outer-edge of the semiconductor wafer 10, it complicates wafer expansion for the purpose of separating the dice comprised in the semiconductor wafer 10 prior to picking up the dice.
Hence, at present, there is a severe limitation in the choice of suitable adhesive mounting tapes to support UV laser wafer singulation applications. This means that an end-user has to adopt newer and much more expensive UV-resistant mounting tapes for this purpose. The change to UV-resistant mounting tape would not only require requalification of the singulation process, but since the UV-resistant mounting tape would be used on multiple tools in the semiconductor assembly and packaging process, requalification of the said process on other tools would also be required. In general, this becomes a very significant barrier for an end-user to consider adopting a UV laser singulation process, even though the singulation results might be superior and more cost-effective.
Even if the end-user decides to use such novel UV-curable and UV laser-resistant mounting tapes, any unintended cuts in the mounting tape is considered undesirable, since the UV-resistant mounting tape may still be punctured and lead to the loss of wafers, either in the singulation machine itself or during subsequent die-picking steps. These risks are further increased when high-power and multi-pass UV laser singulation processes are used.
Furthermore, in laser-singulation applications where multiple cutting lines are cut simultaneously, it is extremely difficult to avoid cutting into the mounting tape. Since the wafer is circular in shape, the various cutting lines that are simultaneously cut have different end points at the edge of the wafer. Independent control over starting and stopping of each laser beam would be required for each cutting line that is cut. It would therefore be advantageous to avoid the need for such precise control for multiple-lane cutting with a UV laser.